Quartz

Lenses and optics for Elm.

This is similar to Heimdell/elm-optics. It's more or less a port of the Lens or Optics library from Haskell.

Lenses can be useful to update deeply-nested records. For example, consider the following data types:

type alias Name =
    { first : String
    , last : String
    }

type alias Person =
    { name : Name
    }

If you want to update a Person's Name, you have to jump through some hoops:

let
    oldName =
        person.name

    newName =
        { oldName | first = String.trim oldName.first }
in
{ person | name = newName }

Updates like this can be performed more directly with lenses. First you'll have to define the lenses:

import Quartz as Qz

nameLens : Qz.SimpleLens l Person Name
nameLens =
    Qz.lens .name <| \ person name -> { person | name = name }

firstLens : Qz.SimpleLens l Name String
firstLens =
    Qz.lens .first <| \ name first -> { name | first = first }

Then you can use the lenses to easily perform the update:

Qz.over (Qz.nameLens |> Qz.andThen Qz.firstLens) String.trim person